1.What is the difference between a process and a thread?

• Process: The basic unit for resource allocation in an OS, with independent memory space and system resources.
• Thread: The basic unit for CPU scheduling, belongs to a process, shares the process’s memory and resources, but has its own stack and registers.
• Differences:
  • Inter-process communication (IPC) is complex; inter-thread communication is simple (shared memory).
  • Thread switching has lower overhead than process switching.
  • Multithreading suits I/O-intensive tasks; multiprocessing suits CPU-intensive tasks.（待研究)
• What is the difference between TCP and UDP?
  • TCP:
    • Connection-oriented, reliable transmission.
    • Provides flow control, congestion control, and sequence guarantee.
    • Suitable for file transfers, HTTP, etc., where reliability is critical.
  • UDP:
    • Connectionless, unreliable.
    • Low overhead, fast.
    • Suitable for video streaming, DNS queries, where real-time performance is prioritized.
• What is the TCP three-way handshake? Why is it necessary?
  • Process:
    1. Client sends SYN (seq=x).
    2. Server responds with SYN+ACK (seq=y, ack=x+1).
    3. Client sends ACK (ack=y+1).
  • Reason:
    • Ensures both sides can send and receive data (bidirectional confirmation).
    • Prevents interference from old connections (using sequence numbers).
    • Initializes sequence numbers for synchronized communication.
• What is the TCP four-way wave?
  • Process:
    1. Active party sends FIN (indicating it stops sending data).
    2. Passive party responds with ACK (acknowledging FIN).
    3. Passive party sends FIN (indicating it also stops).
    4. Active party responds with ACK (confirming closure).
  • Note:
    • Passive party may have unsent data, so closure is split into two steps.
    • TIME_WAIT ensures the final ACK is received, avoiding connection issues.
• What is the difference between HTTP and HTTPS?
  • HTTP:
    • Plaintext transmission, port 80.
    • No encryption, vulnerable to eavesdropping or tampering.
  • HTTPS:
    • HTTP+SSL/TLS, port 443.
    • Provides encryption, authentication, and data integrity.
    • Requires certificates, adds handshake overhead.
• What is the DNS resolution process?
  • Process:
    1. Local query: Check browser cache, system hosts file.
    2. Query local DNS server (recursive resolver).
    3. Local DNS queries root server for top-level domain server address.
    4. Query top-level domain server (e.g., .com) for authoritative server address.
    5. Query authoritative server for target IP.
    6. Return result to client, cache for faster future queries.
  • Types: Recursive query (client to local DNS), iterative query (local DNS to root/authoritative servers).
• What is the ARP protocol? How does it work?
  • Definition: Address Resolution Protocol, maps IP addresses to MAC addresses.
  • Working Principle:
    1. Sender broadcasts ARP request with target IP.
    2. Target host responds with ARP reply containing its MAC address.
    3. Sender caches IP-MAC mapping for future communication.
  • Scenario: Used in LANs, where the link layer requires MAC addresses.
• What is congestion control? How does TCP implement it?
  • Definition: Prevents network overload by controlling sending rates.
  • TCP Implementation:
    • Slow Start: Exponentially increases sending window to probe network capacity.
    • Congestion Avoidance: Linearly increases window to avoid rapid growth.
    • Fast Retransmit: Retransmits lost packet immediately after 3 duplicate ACKs.
    • Fast Recovery: Reduces window but avoids slow start.
  • Algorithms: Reno, Cubic, etc.